1. Field of the Invention
The present invention relates to an electrophotographic (EP) photoreceptor within an EP printer. More particularly, the invention relates to authenticating and determining the thickness of an electrophotographic photoreceptor incorporating a spectral marker.
2. Description of the Related Art
An EP printer, such as a laser printer, is comprised of a print engine and a replaceable EP process cartridge. The replaceable process cartridge supplies toner, as well other wearing components necessary for the electrophotographic process. The photoreceptor is generally thought of as a replaceable supply, but may be found within either the print engine, or the process cartridge.
The useful life of an EP supply cartridge typically runs from a few thousand, to several tens-of-thousands of prints, while a print engine may have a rated life in the hundreds-of-thousands of prints. Printer original equipment manufacturers (OEMs) develop print engines and process cartridges in tandem, since they must work together to maintain image quality throughout the life of both of these printer components. Printer development typically includes: (1) optimization of out-of-box properties for the print engine and process cartridge; (2) a comprehensive study of how image quality changes throughout the life of the print engine and process cartridge.
FIG. 1 shows the voltage vs. exposure energy curves for photoreceptors 10 and 12, as measured on an in-house electrostatic tester described herein. The electrostatic tester generates a plot of photoreceptor voltage as a function of laser exposure energy. The resulting curve is called a photoinduced discharge (PID) curve. The PID curves show the level of electrical contrast between exposed and unexposed regions of the photoreceptor. Lower exposure energies are therefore used when printing lower optical densities, such as halftone images. Higher exposure energies are used when printing darker densities, such as all black images. The initial electrostatics between photoreceptors 10 and 12 are similar at low exposure energies, but separate at higher energies. The properties of EP components, such as the EP photoreceptor, must also be studied over the life of the print engine and process cartridge. FIG. 2 plots the photoreceptor voltage over 50,000 (50 k) prints for photoreceptors 20 and 22 as measured in a printer. The lines show how the electrostatics from an all black page, using the factory preset print mode, change with printing. The zigzagging lines trace the photoreceptor electrostatics at the beginning (cold) and end (hot) of a series of five 10,000 (10 k) print runs. The sloped lines represent the hot-cold fatigue arising from electrostatic measurements taken at the end, and beginning of a 10 k print run. Photoreceptor 20 undergoes negative fatigue, both hot-to-cold, and fatigue over the 50 k prints. Negative fatigue describes a photoreceptor that shows a lower degree of discharge at a given exposure energy, with usage. Conversely, positive fatigue describes a higher degree of discharge at a given exposure energy, with usage. Photoreceptor 22 shows positive fatigue, both hot-cold, and fatigue over the 50 k prints usage. FIG. 3 plots the electrostatics for photoreceptor drums 30 and 32 printed at an all black optical density, using a toner saving run mode. The optical densities are generally lower than the standard preset mode. Photoreceptor 30 shows negative fatigue, both hot-cold, and fatigue over 50 k prints. Photoreceptor 32 shows more stable electrostatics, both hot-cold, and over 0-50 k prints.
Once component fatigue patterns are modeled, optical density (OD) compensation schemes may be developed to offset OD changes as a function of usage. These methods typically involve either predictive algorithms, or toner patch sensing. Printer usage is measured and stored in a number of different ways, including: page counting, pel counting, toner paddle torque sensing, and EP component cycle counting. Predictive algorithms employ usage measurements to change one or more EP operating points to maintain substantially constant image properties. Toner patch sensing operates by creating a toner patch on the photoreceptor with a set of predetermined EP operating points to give a theoretical patch OD. A light source, such as an LED, irradiates the surface of the photoreceptor with a beam of light. The light source is typically an IR emitter, since carbon black, a component of black toner, absorbs strongly in this range. A sensor or detector, such as a photodiode, measures the intensity of the reflected light. Reflectance data is then related to the OD of the patch. The measured OD of the patch is compared to the predicted value stored in the printer. EP operating points are changed to reflect whether the patch was lighter or darker than the predicted value. The print optimization work that enables these compensation schemes is done using process cartridges designed for specific print engine families. If one or more electrophotographic components are substituted for, such as the EP photoreceptor, the utility of these methods become compromised and may result in poorer quality images.
Toner patch sensing attempts to compensate for light or dark print by changing EP operating points. FIG. 4 shows the PID curves for photoreceptors 40 and 42. Assume that the print engine/process cartridge optimizations were designed around photoreceptor 40, but the current process cartridge contains photoreceptor 42. This situation may arise when using an aftermarket photoreceptor that has not been optimized for use within this printer. Print density as a function of exposure energy will be significantly lower for photoreceptor 42 than the toner patch system predicted. The compensation scheme will attempt to increase the OD of the print by, for example, increasing the laser power and/or laser pulse width. Changing EP operating points in the extreme could decrease print speed, diminish image quality, generate excess heat, and decrease component life. In other words, the electrical properties of the process cartridge can adversely affect the life of components within the print engine. A higher rate of component degradation may not become apparent for quite some time. A lower print engine life may not, therefore, be correlated to the process cartridge by the user. Since EP photoreceptors and EP printers are developed in tandem by the OEM, a means for recognizing the OEM photoreceptor within a printer family would be of great use. For example, authentication of an OEM photoreceptor by that OEM's printer could be a prerequisite for employing OD compensation schemes. Additionally, information stored within the EP printer would be useful to the OEM when trouble-shooting print engine failures.
Systems designed to maintain image quality do not typically measure physical properties of EP components directly. Predictive algorithms do not require real-time input, and toner patch sensing measures the amount of developed toner on an imaging surface. Knowledge of the EP component properties, such as the thickness of the photosensitive layer, would be useful. For example, the end of cartridge life in some EP printers is determined when a narrow black band is observed at the left edge of the paper. This band occurs at photoreceptor frequency and is caused by high photoreceptor wear, often at the drive side of the printer. When a portion of the photoreceptor coating becomes too thin to hold charge, this region develops toner that is transferred to paper. FIG. 5 shows an example of this cartridge end-of-life mechanism where toner 500 has been transferred to the paper 501. Thickness data in high wear areas could be used to warn the user when the photosensitive layer thickness was becoming too thin to hold charge. Determining the photoreceptor layer thickness within a printer would also be of interest as an input for print density compensation schemes. FIG. 6 shows the PID curves for three photoreceptors 60, 62, and 64 which differ only by the thickness of the charge transport layer. The differences arise from a complex interaction of capacitance, which is inversely related to thickness, and charge transit time, which is proportional to thickness. Capacitance dominates the lower exposure energy region, while charge transit time dominates the high energy region. Capacitance also affects transfer of toner from the photoreceptor to a receiving substrate, such as paper.
Information regarding the photoreceptor may be stored in either the print engine or process cartridge. Data storage methods within EP print engines and process cartridges are well known in the EP arts. For example, data may be stored in the computer of the print engine, or in a memory element within the process cartridge. The same elements which control EP operating points may also use photoreceptor authentication and thickness determinations as inputs. Modern EP printers typically contain data transfer circuits, and system cards comprising: microprocessors, digital signal processors, controllers, as well as other stored program processors. These elements may also be used for calculations and conversions.
In a general sense, the photoreceptor is called upon to create (with the image writing light source), develop, and transfer a latent image to a substrate. The physical properties of the photoreceptor are critical for the performance of these tasks. As such, determining properties of the photoreceptor, such as the photosensitive layer thickness, is important to characterize the current state of print quality in an electrophotographic printer. A method for measuring the photosensitive layer thickness in an EP printer represents an unmet need within the electrophotographic arts.
To summarize, the present invention addresses at least two unmet needs associated with EP printers: (1) OEM photoreceptor authentication within an EP printer; (2) thickness determination of an OEM photoreceptor within an EP printer.